Stress and Refractive Index Control of SiO₂ Thin Films for Suspended Waveguides

Film stress and refractive index play an important role in the fabrication of suspended waveguides. SiO₂ waveguides were successfully fabricated on multiple substrates including Si, Ge, and Al₂O₃ wafers; the waveguides were deposited using inductively coupled plasma chemical vapor deposition at 100 °C. The precursor gases were SiH₄ and N₂O at 1:3 and 1:9 ratios with variable flow rates. The occurrence of intrinsic stress was validated through the fabrication of suspended SiO₂ bridges, where the curvature of the bridge corresponded to measured intrinsic stress, which measured less than 1 µm thick and up to 50 µm in length. The flow rates allow film stress tunability between 50 and -65 MPa, where a negative number indicates a compressive state of the SiO₂. We also found that the gas ratios have a slight influence on the refractive index in the UV and visible range but do not affect the stress in the SiO₂ bridges. To test if this method can be used to produce multi-layer devices, three layers of SiO₂ bridges with air cladding between each bridge were fabricated on a silicon substrate. We concluded that a combination of low temperature deposition (100 °C) and photoresist as the sacrificial layer allows for versatile SiO₂ bridge fabrication that is substrate and refractive index independent, providing a framework for future tunable waveguide fabrication.